1. Field of the Invention
The present invention is generally related to actuators which are used in plunger-type switches and, more particularly, to actuators which incorporate a sphere within a distal end of the actuator with a low friction insert disposed beneath the sphere to increase the life and improve the wear characteristics of the actuator.
2. Description of the Prior Art
Limit switches have been used in many applications for many years. Likewise, limit switches which utilize a plunger actuator are also very well known to those skilled in the art. Although the plunger actuator of this type of switch can have a plain distal end, actuators of this type often are provided with some means of alleviating potential frictional damage at the distal end of the plunger that could possibly caused by relative movement between the end of the plunger and an external actuating device in a direction which is not parallel to the operating movement of the plunger. In other words, since the plunger actuator is usually intended to move into and out of a switch housing along a direction parallel to the central axis of the plunger, movement in directions which are not parallel to that central axis could cause wear damage at the end of the plunger which contacts the external actuating device. For many years, this frictional wear problem has been addressed by providing either a roller mounted to the distal end of the plunger by an axle means or, alternatively, by a ball bearing disposed in an opening formed at the distal end of the plunger. Each of these solutions finds use in various applications of this type of limit switch.
It is not uncommon to apply a switch in a manner which causes wear of its actuator. U.S. Pat. No. 2,487,922, which issued to De Chant et al on Nov. 15, 1949, describes a curb switch which utilizes a spring-like actuator with a ball at its end. The device is mounted to an automobile for the purpose of indicating proximity between the automobile and an object such as a curb. When the ball at the end of the actuator contacts the curb, the actuator is depressed and a switch mechanism is operated. In this particular case, the ball is not rotatable relative to the actuator.
U.S. Pat. No. 3,300,597, which issued to Hewett on Jan. 24, 1967, discloses a limit switch that includes a spring-load push type plunger rod that is provided with an improved contact head which is designed in such a way that it effectively causes operation of the push rod when contacted at any point through 360 degrees. The switch is relatively easy to install because is does not require orientation in any particular direction. The actuator has a pyramid-shaped device attached to its end to allow downward movement of the actuator plunger in response to contact from virtually any angle in a plane perpendicular to the central axis of the plunger.
When subjected to light duty, limit switches which utilize a ball bearing plunger actuator work in a generally satisfactory manner and exhibit relatively long lifetimes. The ball bearing rotates within an opening of the actuator and alleviates the problems that might otherwise be caused by wear of the distal end of the plunger. However, certain applications require that depression of the plunger into the switch housing be counteracted by an internal spring having a relatively high spring constant. For example, while light duty switches of this type normally require a force of approximately 3 to 6 pounds to actuate the switch by pressing the plunger into the housing, certain applications require that a force of 6 to 12 pounds be utilized to actuate the switch. The reasons for this increased force requirement relate to a need to assure that the switch isn't inadvertently actuated by a relatively slight contact against the plunger and, in some cases, there is a requirement that sustained actuation of the plunger does not permit the plunger to be improperly retained in the actuated position following release of the actuating device as a result of freezing of water or other liquids around the depressed plunger. To counteract this freezing problem, an internal spring with a relatively high spring constant is used to urge the actuator out of the switch housing and away from its actuated position with a force of approximately 6 to 12 pounds. It has been determined that a force of this magnitude is usually sufficient to counteract any restrictions caused by icing at the outside regions of the switch.
These heightened requirements for plunger switches with increased actuation force requirements cause several problems. For example, the force against the distal end of the plunger, needed to counteract the stronger internal spring, is more likely to create wear at the distal end than in the case of a switch with a weaker spring. In addition, the device used to actuate the switch can also experience wear from contact with the plunger under these higher force conditions. It should be understood that, even in applications where the actual number of switch actuations is relatively small, operations which require continually maintained depression of the plunder can create severe wear problems because the distal end of the plunger is in constant contact with the actuating member, under the higher force conditions, for long periods of time during which vibration can cause relative movement between the plunger and the actuating member. For example, if the switch is used in an application wherein a door closure depresses the plunger, continued operation with the door closed will possible damage either the door or the actuator if continued vibration occurs under these high force conditions. This is particularly possible in situations where mobile vehicles use this type of switch to indicate that a door is closed. As the vehicle moves over rough terrain, vibrations and bouncing can cause relative motion between the door and the distal end of the plunger even though relative movement between these components comprises relatively short distances. The vibratory nature of this contact under the high force of the spring within the switch housing will significantly degrade the operation of the switch and reduce its lifetime.
In view of the requirements described above, it would therefore be extremely beneficial if a ball bearing plunger actuator is provided which can withstand the higher forces required of the spring within the switch housing while permitting continued rolling contact between the distal end of the plunger and a device used to activate the plunger by moving into contact with it.